Our ds almost 5 has also just had scarlet fever and it started in just the way you describe. apparently it is now uncommon in GB but seems endemic here (CH) and he also caught it at school.
We were advised that he would not be contagious after just 24 hours of antibiotics, but did delay our departure for ski holiday (fortunately we were driving), especially as he also had an ear infection, a further 24 hours to be safe. By this time he was fine in himself and the most unsightly reddened patches around his nose and mouth disappeared within a few days so I would take him out. Do keep applying a good moisturiser though as this seemed to work well on our ds' sensitive skin.
Incidently our dd (almost 18 months) has not contracted it from her brother and is now beyond the incubation period, so our doctor's advice that she would probably avoid it was correct.
hope this helps
LizS